PURPOSE: To make a flow of combustion gas as one flow and simplify a structure by a method wherein fire tubes are inserted into water pipes of a vertical water pipe boiler, an eccentric flow of combustion gas is eliminated by applying the fire tubes as passages for the combustion gas and at the same time a heat transferring promotor is arranged in each of the fire tubes.

CONSTITUTION: Annular headers 1 and 2 are arranged at an upper part and a lower part. Water pipes 3 are arranged between these headers. Fire tubes 4 are inserted into the water pipes 3. The water pipes 3 are arranged in an annular form so as to form a water-cooled wall of a combustion chamber 5. In addition, a burner 6 is installed at an upper part of the combustion chamber 5, and a combustion gas passage 7 communicating with the fire tubes 4 is arranged at a lower part of the combustion chamber 5, respectively and further a fire chamber 8 communicating with a chimney is arranged at an upper part of the upper header 1. In turn, A down pipe 9 for use in naturally circulating boiler water is arranged outside the water pipes 3. A swirl tape is inserted into the fire tubes 4 and a heat transfer coefficient from combustion gas within the tubes into the fire tubes 4 is promoted. With such an arrangement as above, a temperature of the combustion gas is lowered with a passage of combustion gas being applied as one upward flow and the gas is applied as a boiler discharged gas.